1. Field of the Invention
The present invention relates generally to a heat treating process for a rod by the use of a fluidized bed, and more particularly to a process for adjusting the strength of the rod wherein the heated rod is variably cooled within the fluidized bed.
2. Description of the Prior Art
Heretofore, heat treating processes for a rod, by the use of a fluidized bed, have been known wherein solid particles, such as, for example, alumina, silicon sand, zircon sand, and the like, are fluidized by injecting a fluid under pressure from below a layer of the solid particles, and a rod is heat treated within the medium of such a fluidized bed. In this respect, the strength of the rod may be varied by varying the cooling rate of the rod as the same passes through the fluidized bed. According to prior art heat treatment processes, the temperature of the fluidized bed is varied for varying the cooling rate of the rod, an increase in the temperature of the fluidized bed resulting in a corresponding decrease in the cooling rate of the rod, and vice versa.
Such prior art heat treatment processes, however, pose the following problems:
In case the temperature of the fluidized bed is increased to approximately 600.degree. C., the thermal strain or deformation becomes evident within the container or tub containing the fluidized bed, which is, for example, made of steel plate, and thus, the same fails to maintain the fluidized bed in a satisfactory condition. For this reason, there arises a need to line the inner surfaces of the tub, made of steel plates, with heat-resisting bricks. The requirement for varying the temperature of the fluidized bed over a wide range extending from room temperature up to 600.degree. C. leads to the use of the aforenoted lining of heat-resisting bricks for the construction of the tube containing the fluidized bed, however, such results in an increase in the manufacturing costs and complexity in construction.
Another shortcoming of such prior art is that solid particles are discharged from the surface of the fluidized bed, and there arises a need for using a dust collector. The heat resisting problem is again confronted in this instance, however, because an increase in the temperature of the fluidized bed necessarily leads to an increase in the heat-resisting temperature of the dust collector.
Another prior art method is known which avoids the aforenoted shortcomings, that is, the temperature of the fluidized bed is maintained at a relatively low level for obviating the need of the aforenoted lining, as well as for lowering the heat-resisting temperature of the dust collector. More specifically, the length of the fluidized bed is maintained constant, while the cooling rate of the rod is varied, and to this end, the speed of the rod passing through the fluidized bed is varied. However, this attempt poses another shortcoming in that the number of rods being heat-treated varies with the speed of the rods being passed through the fluidized bed.
A solution to this problem may be that the length of the fluidized bed is varied for varying the cooling rates of the rods, however, this dictates the use of several fluidized beds having varying lengths, or a single fluidized bed whose length may be varied, the latter case particularly resulting in complex construction. In addition, a limitation is imposed upon the location of pipes or tubes, such as, for example, radiant tubes for use in increasing the temperature of the fluidized bed.
It has thus been a long-sought goal arising from industry to provide a heat treating process which avoids the aforenoted shortcomings experienced with the prior art, for adjusting the strength of rods within the particularly noted manufacturing environment.